Vectors question bit confused

[rare ferry]

This is really simple but I'm just stuck. I just don't understand how they worked out BM

[formal osprey]

1. Wait patiently for a helper to come along.
2. Once someone helps you, say thank you and close the thread with:

+close

3. Feel free to nominate the person for helper of the week in #helper-nominations
4. Do not ping the mods, unless someone is breaking the rules.
5. If you're happy with the help you got here, and the server overall, you can contribute financially as well:

[summer pawn]

B to C would be -b - a + b

So the bs would cancel out

Then you half that

to get B to M

[rare ferry]

wait how did u get BC

[summer pawn]

one sec

Since O is the centre

From the other edge to O is also a

[rare ferry]

ahhhh

thank you so much

[summer pawn]

np

[rare ferry]

+close